Increase Alphanumeric VARCHAR Entry by Value 1?

On an old project because of not thought through design I have a column which actually should be set to auto_increment, though it cannot be because it are alphanumeric entries as follows:

c01
 c02
 c03
 

(c99 would continue to c100 and more), the letter happened in the past and it would require to overhaul the system to take it out, thus I rather prefer this workaround.

Now I need a way to imitate the auto_increment functionality with the SQL statement myself, my own attempt has gotten as far as the following:

INSERT INTO tags (tag_id, tag_name, tag_description, added_by_user_id, creation_date, last_edited) VALUES (SELECT(MAX(tag_id)+1), 
 
 'Love', 'All about love', 7, now(), 0);
 

This one does not work as is, though the idea was to select the highest entry in the column "tag_id" and then simply increase it by the value 1.

Any ideas how to accomplish this?

By the way I am also not sure if you simply can increase an alphanumeric entry through this way, though I know it can be done, I just don't know how.

---